In her madcap youth, Pamela Stephenson won notoriety for a series of ribald pranks. To promote her career in comedy, she made a habit of behaving badly in public: stripping under a tablecloth in a crowded restaurant; gleefully interrupting one chat show by attempting to remove the host's trousers, and another by wrapping her legs around her interviewer's neck.

A favourite of the tabloids, she fed their appetite for outrageous antics. She dressed as a nun armed with a chainsaw, donated a gold G-string to a charity auction called What the Stars Wear at Home, and joined Diana, Princess of Wales, in disguise as a policewoman at the Duchess of York's hen party. But all that happened 20 years ago, when curvy Stephenson was a rising star on stage and screen (Not the Nine O'Clock News, Superman III and Saturday Night Live). That wild, bubbly creature who fell in love with the even wilder Billy Connolly and became his wife at the end of the 1980s has transformed herself almost beyond recognition.

Sitting across from her in the refined lounge of the Four Seasons Hotel in Beverly Hills, I am hard-pressed to find much resemblance between the giggly, tabloid Stephenson of old and the dignified, soft-spoken character dressed in black who rarely cracks a smile and tells me that she now prefers to be known as Professor Connolly.

It's not a joke. At 53, she is in full flight from her younger self, having abandoned comedy for a career as a clinical psychologist in Hollywood and a professor of psychology at the California Graduate Institute, a respected college in Los Angeles. Her speciality is sex research and she is an authority on the subject in academic circles.

She knows, however, that there is a lurking suspicion in Britain and Australia that she is still playing tricks on the world and only pretending to be a sober academic. "They still think I'm a wacky comedian and they have trouble accepting that I've become something entirely different. There seems to be this impression that if I really am a psychotherapist, I can't be serious about it. They think there must be something fishy going on."

Dr Connolly - the child of two professors in Australia, she earned her own doctorate several years ago - is wary of discussing either her work as a therapist or her research. She modestly says that she lectures and presents papers on such things as helping transgendered people cope with the world.

More controversial is her study of sadomasochists and bondage, which is well known to her peers but not to the outside world. In the Los Angeles S&M community, she has been hailed for conducting "groundbreaking" research with clinical subjects she recruited and interviewed at the Club Laire De Sade in North Hollywood.

Earlier this year, she gave a paper at a conference, in which she made the case that sadomasochists are basically ordinary people whose psychological profiles are not much different from yours or mine. For her willingness to defend the community, her new S&M fans have praised her as a champion of kink (to use their word). In November, she will chair a panel at the Society for the Scientific Study of Sexuality, in Pennsylvania, titled "Who's Afraid of the Big, Bad Sadomasochist? Redressing Sadomasophobia".

All of which is interesting, given that what she is most famous for now is her best-selling book about the childhood of her husband, who seems to have spent his early years being beaten by everyone in sight, from his aunts to his father, who also abused him sexually. In fact, as she readily admits, Billy - the biography - is really a work of psychology, as is her latest book about the comedian, Bravemouth.

At home, she has found her perfect research subject. What's strange, however, is that her academic defence of sadomasochists appears at odds with her moving accounts of Connolly's efforts to overcome his abuse. The difference between the two, it would seem, is that her S&M subjects are only playing rough - as they see it - whereas Connolly got the real thing.

And she has no sympathy for the real thing. "So many people suffer from abuse and suffer alone. Billy's story has helped some of them to know that they're not alone or lost. But I've really written my books for my husband and our family. They've brought us closer together by allowing us to discuss things that were unspoken for so long. It has been great to show how Billy tried to make sense of his life. It enabled him to mourn his past and to heal himself. At some point, somebody was going to have to tell his story and I thought it might as well be me."

The result is that she's had Billy on the couch for analysis over the past few years, but only in a metaphorical sense. He won't sit still long enough. "I've had to interview him on the move. Among other things, he has attention deficit disorder - which is a personal problem, but is also the strength of his comedy - and I can get only a few answers from him at a time. He's like quicksilver, always slipping away."

She talks a lot of overcoming obstacles and helping Connolly to know himself, but you don't get the impression she's content. She still seems to be searching for answers and is defensive about her search, worried about the prying eyes of the tabloids and full of lingering doubts that her new work will be taken seriously in the outside world.

In one respect, the manic touch is still there. For most people, it would be enough to write one book about a spouse. But she has followed it with another and could easily write a third. "It was so satisfying to write the first book that I didn't want to let Billy go."

If she can't have him to herself all the time, she can at least capture him on paper. It sounds like the making of an epic obsession. And she doesn't deny it.

"Love is an obsession. It has that quality to it. But there are healthy obsessions and mine is one of them."

Bravemouth: Living with Billy Connolly, by Pamela Stephenson, will be published by Headline on Saturday.